The present invention relates to the forming of three-dimensional micro- and nanostructures, in particular optical elements of the refractive or hybrid refractive/diffractive type.
More specifically, the invention relates to a method for forming three-dimensional micro- and nanostructures based on the structuring of a body of material, for example a plastics material, by means of a mould having an impression area which reproduces the desired three-dimensional structure in negative form, and a set of microlenses with a convex three-dimensional structure which can be produced by means of this method.
The reduction of the size of devices in the fields of microelectronics, optics, micro- and nanomechanics, microfluidics, and sensor technology—to quote only a few examples—has presented increasing challenges in relation to the improvement of the resolution, accuracy, flexibility and reliability of these devices, and has stimulated research into new micro- and nanofabrication methods.
To quote one example, in the optical field in particular, the fabrication of three-dimensional nanostructures can allow the production of refractive optical elements which can be substituted for the diffractive optics currently used in systems for generating and processing complex images. This would make it possible to overcome the drawbacks of the use of diffractive optics, these drawbacks including an efficiency which is usually below 50%, the generation of disturbances due to orders of diffraction differing from those chosen to implement the desired optical function (“ghost images”), and the necessity of using monochromatic sources having a predetermined emission wavelength, correlated with the pitch of the diffraction grating.
The fabrication of complex three-dimensional micro- and nanostructures, such as convex structures for forming refractive optical elements (microlenses), requires the use of high-precision substrate micro- and nanoprocessing methods.
There are known processing methods based on etching and material removal by means of focused ion beams (“FIB”), which are fairly flexible in respect of the forming of the desired shapes, but are very slow and therefore not suitable for the forming of complex and extensive structures.
There are also known optical lithography or “grey-tone” electron beam methods, whose writing speed is sufficient for the required applications, but which have a low depth of penetration into the materials (typically electronic resists) being processed, and which therefore cannot be used to produce three-dimensional structures having depressions or projections measuring more than a few microns.
One emerging technology is that of nano-imprint lithography (“NIL”), which makes it possible to dispense with the use of beams of energetic particles, such as photons, electrons or ions, for printing a given relief pattern on a plastics material, for example a polymer film.
The general principle of nano-imprint lithography is that of replicating a relief pattern present on the surface of a mould by pressing the latter on a body of mouldable material which is deposited on a substrate, and which can be deformed by pressure. The material tends to fill the cavities of the mould and to match its profile. Finally, the mould is removed, leaving its profile imprinted in the material.
The nano-imprint lithography method, like other methods of forming a plastics material (such as injection moulding) requires a mould which is formed with absolute precision, particularly as regards its imprint area carrying the microstructured or nanostructured profile which is to be replicated in the plastics material.
Advantageously, when the mould has been produced, it can be used in serial production without the need to repeat complex stages of lithography or direct etching of each substrate on which the desired structure is to be fabricated.